Kinetic coupling and hydrogen surface fugacities in heterogeneous catalysis: I. Alkane reactions on Te/NaX, H-ZSM5, and Ga/H-ZSM5

Journal Article · Wed Apr 01 04:00:00 EST 1992 · Journal of Catalysis; (United States)

DOI:https://doi.org/10.1016/0021-9517(92)90342-F· OSTI ID:7017204

Iglesia, E; Baumgartner, J E; Price, G L ^[1]

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1. Exxon Research and Engineering Corp., Annandale, NJ (United States)

Hydrogen removal occurs by recombinative desorption and by hydrogen transfer during dehydrogenation steps required for alkane and cycloalkane conversion on Te/NaX, H-ZSM5, and Ga/H-ZSM5 catalysts. Recombinative desorption limits the rate of n-heptane and methylcyclohexane aromatization on Te/NaX and prevents equilibrium between gas-phase H[sub 2] and H-adatoms formed in intermediate dehydrogenation steps. The resulting high surface hydrogen fugacities lead to low steady-state concentrations of required unsaturated intermediates. Te ions catalyze rate-limiting hydrogen desorption steps during alkane reactions on Te/NaX. On H-ZSM5, hydrogen removal limits the rate of propane conversion to aromatics. Hydrogen adatoms are removed predominantly by reactions with coadsorbed hydrocarbon fragments, leading to high cracking selectivity. Ga ions introduce a recombinative desorption function that partially relieves the resulting high hydrogen surface fugacities and allows dehydrogenation steps to occur without concurrent cracking. Thus, Ga ions increase aromatics selectivity by providing a porthole for the removal of hydrogen adatoms as dihydrogen. The authors propose that rate-limiting hydrogen desorption steps, and the high surface hydrogen fugacities that result, control the rate and selectivity of dehydrogenation and related reactions on many nonmetal surfaces.

• OSTI ID: 7017204
• Journal Information: Journal of Catalysis; (United States), Journal Name: Journal of Catalysis; (United States) Vol. 134:2; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020400 -- Petroleum-- Processing
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ALKANES
AROMATICS
CATALYSIS
CATALYTIC EFFECTS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CYCLOALKANES
CYCLOHEXANE
DEHYDROGENATION
DESORPTION
ELEMENTS
GALLIUM
HEPTANE
HETEROGENEOUS CATALYSIS
HYDROCARBONS
HYDROGEN
HYDROGEN TRANSFER
INORGANIC ION EXCHANGERS
ION EXCHANGE MATERIALS
KINETICS
MATERIALS
METALS
MINERALS
NONMETALS
ORGANIC COMPOUNDS
PHYSICAL PROPERTIES
PROPANE
REACTION INTERMEDIATES
REACTION KINETICS
SEMIMETALS
SILICATE MINERALS
SORPTION
SYNTHESIS
TELLURIUM
THERMODYNAMIC PROPERTIES
VAPOR PRESSURE
ZEOLITES